The present disclosure relates to a cam mechanism converting rotational motion to linear motion, a fixing device including the cam mechanism and an image forming apparatus including the fixing device.
An image forming apparatus is provided with a fixing device which heats and presses a toner image transferred on a sheet and fixes the toner image on the sheet. The fixing device includes a heating roller heated by a heat source and a pressing member being pressed against the heating member to form a pressing area through which the sheet passes. In order to remove the sheet jammed at the pressing area or in order to make the sheets having different thicknesses pass through the pressing area, the fixing device is sometimes made so as to vary a pressure of the pressing area. In such a fixing device, the pressing roller is made to move in directions close to or away from the pressing roller to vary the pressure of the pressing area. As a mechanism to move the pressing roller, a cam to convert rotational motion output by a drive source, such as a motor, to linear motion is used.
In the above fixing device, the pressing roller abuts on the cam by a biasing member. Thereby, a direction of load torque applied to the cam by the biasing member is changed from a rotating direction of the cam to a counter rotating direction between an ascending region and a descending region of the cam. Then, the cam may be shifted (jumping movement) by ratting of each member regardless of a driving force of the motor, and various problems, such as noise and vibration, may occur.
In addition, when a radius of the cam is varied in the ascending region and the descending region, a force applied to the cam from the biasing member is varied. Thereby, the motor rotating the cam is required to have sufficient output force for both acceleration and deceleration. Additionally, because an input current for the motor increases and decreases during the rotating of the cam, the motor is required to have capacity capable of withstanding the change of the input current.